1. Field of the Invention
The present invention relates generally to the field of radar and, more particularly, to radar apparatus having provision for substantially reducing background clutter, especially sea clutter, at low radar grazing angles.
2. Description of Related Art
Radar reflections (echos) from irregular background surfaces, especially at low radar grazing angles, are known to interfere, often drastically, with the detection of low target objects such as ships and low flying aircraft and missiles. Because of the importance of being able to quickly detect targets at substantial ranges, much effort has understandably been directed towards target enhancement and/or background clutter suppression.
Both range gating and Doppler filtering or Doppler frequency discrimination are known techniques for reducing the relative effects of background clutter in radar systems. With particular respect to Doppler filtering, it is well known that frequency shifts, known as Doppler frequency shifts, are obtained from radar returns from moving objects. Since radar returns from objects that are stationary, or are stationary with respect to the radar transmitter and receiver, do not exhibit Doppler shifts, the possibility exists, through Doppler frequency filtering, to discriminate moving objects from stationary objects and, hence, moving targets from stationary background.
Ground and sea clutter are, however, very widely dispersed and range and Doppler ambiguities greatly compound the problem of discriminating targets from their background. Range ambiguities, in effect, break the range profile of radar target and background echos into zones which are superimposed upon one another. As a consequence, the radar echo from a target may be received simultaneously with clutter not only from the target's own particular range but also from the corresponding range in every other range zone. An unambiguous range zone, in nautical miles, is approximately equal to 80 divided by the radar pulse repetition frequency (PRF) expressed in kHz; therefore, increasing the radar PRF narrows the zone ranges and increases the number of superimposed zones, making it increasingly difficult to isolate the target echo from the background clutter.
On the other hand, Doppler ambiguities cause successive repetitions of the target and background Doppler profile to overlap, resulting in a target echo having to compete with background clutter whose true Doppler frequency is quite different from that of the target. In contrast to range ambiguities, increasing the radar PFF has the effect of moving the successive repetitions of the main lobe clutter further apart on the frequency axis, thereby making isolation of the target echo easier.
Thus, at low radar PRF's, Doppler ambiguities dominate, whereas at high radar PRF's, range ambiguities dominate. The selection of radar PRF so as to reduce the effects of background clutter can, therefore, be seen not to be entirely effective.
The polarization state of transmitted and reflected radar signals is also known to have an effect on the ability to discriminate targets from background clutter. In brief, radar signal polarization can be understood by considering the radar signals, both transmitted and reflected to comprise electromagnetic waves having a transverse oscillatory motion defined by the orientation of the associated electric field vector. In the general polarization case, the terminus of the electric field vector traces out an elliptical path in space, the sense of the polarization being either right- or left-handed according to the rotational direction of the electric field vector as viewed along the direction of wave propagation. This general case is known as elliptical polarization. Linear and circular polarization are seen to be special cases of elliptical polarization.
Elliptically polarized transmitted signals are provided by shifting the phase between the horizontal and vertical components of the signal. Ordinarily, the receiving antenna is geometrically similar to the transmitting antenna and, in fact, the same antenna is usually used, on a time sharing basis, as both a transmitting and a receiving antenna. Radar return signals are commonly processed so that it appears that the transmitted and reflected signals have the same polarization state. In such regard, it is, however, generally known that the maximum amount of energy can be extracted from a reflected signal when the polarization state of the return signal is the same as the "polarization" of the receiving antenna. Conversely, when the polarization state of the reflected radar signal is opposite to the "polarization" of the receiving antenna, a minimum amount of energy is extracted from the return signal. Advantage of such effect is taken, for example, by using circularly polarized radar signals or receiver antenna processes to reduce the radar return clutter from rain. It should further be observed that depending upon characteristics of the target, the polarization state of the reflected signal may not be the same as that of the transmitted signal; that is, a target may "depolarize" the signal.
When investigating the possibility of using specific transmitting and signal processing polarization states to assist in target discrimination, either by enhancing the target return signal or suppressing background clutter, it may be advantageous to construct a visual representation of the polarization states involved. One such visualization that may help in the understanding of polarization processing is the polarization sphere of Poincare.
An elliptically polarized wave can be constructed of two orthogonal electric field vectors which represent minor and major axes of the ellipse and which have a ratio, r, of minor to major axes. Moreover, the major axis will be spatially oriented at an angle, .phi., relative to the local horizontal. The polarization of this signal is represented by a single point on the surface of the Poincare sphere. On such a sphere, the point representing the polarization state has a longitude coordinate of 2.phi. and a latitude coordinate of 2.gamma. (wherein .gamma. is the ellipticity angle and is defined as 2.gamma.=2 arctan (r), the two angles .phi. and .gamma. completely specifying the state of polarization. The result is that there is a one-to-one correspondence such that each point on the Poincare sphere represents a specific polarization state and different points on the sphere represent different polarization states.
A discussion of Poincare sphere representation of polarization states can, for example, be found in an article entitled "Virtual Polarization Adaptation" by A. J. Poelman which appeared in IEE Proceedings, Vol. 128, No. 5, pages 261-270, October 1981.
It can be readily determined that all linear polarization states lie on the equator of a Poincare sphere and that circular polarization points lie at the poles of the sphere, depending on whether the circular polarization is right- or left-handed. Points representing all right-hand sense polarizations, including right-hand sense circular polarization, have both .phi. and .gamma. positive and, therefore, lie on the upper hemisphere of a Poincare sphere. Points representing left-hand sense polarizations (both .phi. and .gamma. negative) lie on the lower hemisphere of the Poincare sphere.
At any particular instant in time, there exists for every radar-visible object, including both targets and such background as ground and sea surfaces, a particular radar signal polarization state which produces a maximum signal return. This particular maximum polarization state can, of course, be represented by a point somewhere on the Poincare sphere. Over a period of time, if the object, for example, changes orientation relative to the radar, the maximum polarization state will typically change, giving rise to a series of maximum points for that particular object on the sphere. Depending on the nature of the object and its movement relative to the radar, these maximum points on the sphere may be quite widely scattered.
There is also some evidence suggesting that widely different objects, such as ships, aircraft and missiles, have widely different patterns of maximum polarization points on the Poincare sphere. It does not, therefore, appear feasible to operate a surveillance radar at a particular maximum polarization state which will enhance the discrimination of all or even most potential targets from background clutter. Even in the unlikely event this were possible for known targets, the possibility exists that previously unencountered targets would not exhibit a similar maximum polarization state enabling them to be discriminated from background clutter.
However, any object, at any point in time, will also have a null polarization state; that is, a polarization of transmitter and receiver producing zero signal return. In fact, any fixed reflecting target has two such null polarization states which are represented by a pair of points on the Poincare sphere. These two points representing null polarizations are understood to be on the same great circle of the sphere on which the two cross polarization nulls lie (wherein the transmitter and receiver are cross polarized).
It has been suggested by Poelman in his above-mentioned article that null polarization techniques may possibly provide an approach to enhancing target detection in the presence of background clutter. Poelman has not, however, to the present inventor's knowledge, publicized radar data that illustrates the clutter rejection methods by considering average positions on the polarization (Poincare) sphere.
Improved radar apparatus, and related improved operating processes, are therefore still presently needed to enhance the ability of radars, particularly those operated over water at low grazing angles such as is common for shipboard radars, to discriminate targets accurately and reliability from background clutter.